Actually, few people take planned dates seriously. As long as the end date of a
project is far in the future (figure 1), we don't feel any pressure and work leisurely,
discuss interesting things, meet, drink coffee, ... (How many days before your last
exam did you really start working...?). So at the start of the project we work relatively
slowly. When the pressure of the finish date becomes tangible, we start working
harder, stressing a bit, making errors causing delays, causing even more stress.
The result: we do not finish in time.

We know all the excuses which caused us to
be late. Of course it's never our own fault.It'is not wrong or right. It's human behaviour.
It is how we function. So don't fight it (we cannot fight our genes) but also don't ignore it.
Accept it and then think what we still can do.

Smart project managers tell their team an earlier date (figure 2). If they do this cleverly, the result may be just in time for the real date. The problem
is that they can do this only once or twice. The team members soon will discover
that the end date was not really hard and they will lose faith in milestone dates.
This is even worse.

The solution for coping with these facts of human behaviour
is to plan in very short increments (figure 3).
The duration of these increments should be such that:

The 'pressure' of the end date is felt right the first day.

The duration of a cycle must be sufficient to finish real tasks.

When I was thinking about these issues, I thought: "Three weeks is
too long: people will not work really focused the first day" and "One week may be felt as too short for finishing real
tasks; people will probably not accept it". So I decided to start with 2-week cycles.

The experience in an actual project, where we got only six weeks to finish completely made us decide to using one-week cycles.
If you cannot even plan a one-week period,
how could you plan longer periods ...?
The results were such, that we will continue using one-week cycles on all subsequent projects.

Note that the pressure in this scheme is much healthier than the real stress
and failure at the end of a Big Bang project (delivery at once at the end).
No stress, however, is boring. Good stress (level of figure 3) gives people the feeling of accomplishment
and is sustainable over many cycles. Bad stress (as in figures 1 and 2) makes us inject
too many defects and is bad for our health.